<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>过渡</title>
    <style>
        .item{
            width: 300px;
            height: 300px;
            border:150px solid red;
            transition:all 2s;
            transition-delay: .2s;
        }
        .item:hover{
            border-radius: 50%;
            border-color: black;
        }
    </style>
</head>
<body>
    <div class="item">

    </div>
</body>
</html>